Process for making propylene homo or copolymers

ABSTRACT

A process for homo or copolymerizing propylene, wherein propylene is polymerized in the presence of a catalyst at an elevated temperature in a reaction medium, in which a major part is formed by propylene. The polymerization is carried in at least one slurry reactor in the presence of liquid propylene at a temperature between 80° C. and the critical temperature of the reaction medium and a catalyst system producing within said temperature range a high productivity and essentially constant isotacticity within wide melt index range.

This application is the natiional phase under 35 U.S.C. §371 PCTInternational Application No. PCT/FI97/00702 which has an Internationalfiling date of Nov. 17, 1997 which designated the United States ofAmerica.

The invention relates to a process for making propylene polymers orcopolymers in propylene medium.

BACKGROUND OF THE INVENTION

Several processes for polymerizing alpha-olefins, for example propylene,are known. Such processes where Ziegler-Natta catalysts are employed,are for example slurry polymerization carried out in a solvent such asn-hexane, bulk or slurry polymerization carried out in a liquefiedalpha-olefin monomer such as propylene and gas phase polymerizationcarried out in a gaseous monomer such as gaseous propylene. Further,combinations of these processes are also known such as slurrypolymerization followed by gas polymerization.

Gas phase processes are advantageous in that recovery and reuse of inerthydrocarbon or monomer is more simple than in slurry processes. The costfor equipment for monomer recovery and reuse is small compared to slurryprocesses. One disadvantage of the gas phase processes is that themonomer inside the reactor is in vapor phase and therefor the monomerconcentration is relatively low compared that of slurry processes. Thisresults in a lower reaction rate. In order to increase the polymer yieldper unit weight of catalyst, it is necessary to extend the residencetime in the reactor by increasing the volume of the reactor.

In a book Y. V. Kissin, Kinetics of Polyolefin Polymerization withHeterogenous Ziegler-Natta Catalyst (1981), p. 10,11,70,71,125 theinfluence of temperature in propylene polymerization with TiCl₃-basedZ-N catalysts has been discussed. The active centers of catalysts havebeen shown to be stable up to 80° C. In a polymerization process carriedout at relatively high temperatures, eg. 70-80° C. and high monomerconcentration, the stage at which the rate of chain initiation and chaintermination are equal, is reached early.

The overall polymer yield of such catalysts is in general low and verycostly ash removal is necessary in the process.

According to EP0417995 a special catalyst for propylene polymerizationat very high temperatures of 150-300° C. is disclosed. The catalyst hasa typical structure which is possible with a claimed organoaluminiumcomponent and a silicon compound. However this process is not practicalbecause the proposed polymerization temperatures are higher than themelting temperature of polypropylene.

Sergeev et al. (Macromol. Chem., 185, (1984), 2377-2385) have observedwith TiCl₄/EB-AlEt₃/EB catalysts a slight increase of isotactic indexwhen passing from 20° C. to 60° C. and a rapid decline above 70° C.Further Spitz and Guoyt (Macromol. Chem., 190 (1989), 707-716) reportedfor MgCl₂/TiCl₄ catalyst that the number of active centers remainsconstant within the range of 50-70° C. Above 80° C. the activitydecreases and the catalyst is deactivated.

In many patent applications it is mentioned that higher temperatures,such as up to 100° C., could be used. However in such publications, forexample EP0438068 and EP0412750, only lower temperatures of 70-80° C.are presented in the examples. Therefore, according to prior art onlylower temperatures of up to 80° C. has been used.

From U.S. Pat. No. 5,093,415 it is known a high temperature (over 100°C.) process employing a special catalyst containing magnesium, titanium,halide and carboxylic acid ester containing two coplanar ester groupsattached to adjacent carbon atoms. However this is a gas phase processand comparative examples at lower temperatures show activity decreaseabove 80° C.

Finnish patent application 954814 concerns a process for polymerizingpropylene in at least one slurry reactor, where the temperature and thepressure are above the supercritical temperature and pressure of thereaction mixture. One of the main advantages of operating undersupercritical conditions is that great amounts of hydrogen can be freelyadded to the slurry reactor because hydrogen readily dissolves into thesupercritical fluid.

SUMMARY OF THE INVENTION

The present invention concerns a multistage process for homo orcopolymerizing propylene, wherein propylene is polymerized in thepresence of a catalyst system comprising a procatalyst component and acocatalyst component, said procatalyst component comprising magnesium,titanium and at least one internal donor compound, at an elevatedtemperature in a reaction medium, in which a major part is formed bypropylene. The present invention is characterized in that thepolymerization is carried in at least one slurry reactor in the presenceof liquid propylene at a polymerization temperature between 80-91° C.and by using a catalyst system where said internal donor compound isslightly soluble, the amount of said slightly soluble donor compound inthe catalyst system being at least 1 w-%. This kind of catalyst systemproduces within said temperature range a high productivity andessentially constant isotacticity within wide melt index range.

According to another embodiment of the invention the catalyst system caninclude at least two internal donor compounds, of which one is slightlysoluble internal donor compound and another internal one donor compoundis easily soluble, and the amount of said slightly soluble donorcompound in the procatalyst is at least 1 w-%.

DETAILED DESCRIPTION OF THE INVENTION

According to the invention it has been found that by using in propylenepolymerization a catalyst system having at least one internal donorcompound which is slightly soluble in eluting agents and by using thisdonor compound in a certain amount a highly stereospecific catalystsystem is obtained which give certain performance between temperaturerange of 80-91° C. First, the catalyst system gives a high productivityand secondly, the catalyst system gives relatively high isotacticityindex, which remains essentially constant although polymers have varyingmelt index. With ordinary Ziegler-Natta catalysts the isotacticity indexis at a lower level and drops when the melt index increases.

Examples of the catalyst systems, which are usable according to theinvention, among others, are generally disclosed for example in Finnishpatents FI86866, FI96615, FI88047, FI88048 and Finnish patentapplication FI963707. These catalysts have been presented for use onlyin relatively low temperatures.

According to this invention a suitable catalyst system comprises aprocatalyst composition prepared from magnesium dichloride, titaniumcompound and at least one internal donor compound having a slightsolubility in hydrocarbons or compounds used as cocatalyst, and aconventional cocatalyst compound. According to one embodiment of theinvention the procatalyst composition is obtained by applyingtransesterification method, which is generally disclosed for example inFinnish patent 88048. The transesterification reaction is carried out atan elevated temperature between a lower alcohol and a phthalic acidester, whereby the ester groups from lower alcohol and phthalic acidchange their place.

MgCl₂ can be used as such or it can be combined with silica, e.g. byabsorbing the silica with a solution or slurry containing MgCl₂. Thelower alcohol used can be preferably methanol or ethanol, particularlyethanol.

The titanium compound used in the preparation of the procatalyst ispreferably an organic or inorganic titanium compound, which is at theoxidation state of 3 or 4. Also other transition metal compounds, suchas vanadium, zirconium, chromium, molybdenum and tungsten compounds canbe mixed with the titanium compound. The titanium compound usually ishalide or oxyhalide, an organic metal halide, or a purely metal organiccompound, in which only organic ligands have been attached to thetransition metal. Particularly preferable are the titanium halides,especially TiCl₄. Preferably the titanation is carried out in at leasttwo steps.

The transesterification can be carried out e.g. by selecting a phthalicacid ester—a lower alcohol pair, which spontaneously or by the aid of acatalyst, which does not damage the procatalyst composition,transesterifies the catalyst at an elevated temperatures. It ispreferable to carry out the transesterification at a temperature, whichis between 110-150° C., preferably between 115-140° C.

The alkoxy group of the phthalic acid ester used comprises at least fivecarbon atoms, preferably at least 8 carbon atoms. Thus, as the ester canbe used for example propylhexyl phthalate, dioctyl phthalate, di-nonylphthalate, di-isodecyl phthalate, di-undecyl phthalate, di-tridecylphthalate or di-tetradecyl phthalate.

According to the invention the slightly soluble internal donor compoundis a C₁-C₆ alkyl ester of organic carboxyl acid, preferably C₁-C₆ alkylester of organic dicarboxyl acid and most preferably diethyl phthalate.It is very difficult to elute diethyl phthalate from solid catalyst withsolvents or eluents. The reason may be that small alkyl groups of thephthalate, i.e. ethyl groups, do not solvate easily with hydrocarbon orhydrocarbon containing solvents. Thus the slightly soluble internaldonor is not removed when the procatalyst compound is treated witheluents or cocatalyst compositions and therefore the stereospecifityremains also at high polymerization temperatures.

Eluents are organic and metalloorganic compounds such as compounds ofGroup 1, 2 or 3 metals containing C₁-C₁₀ alkyls. Preferably, the eluentis a metal compound containing C₁-C₁₀ alkyls, which are used also as acocatalyst. The preferable eluent is tri-C₁-C₆-alkylaluminium, morepreferably tri-C₁-C₄-alkylaluminium, most preferably triethylaluminium.

According to one embodiment of the invention propylene is polymerized inthe presence of a catalyst system comprising a procatalyst component anda cocatalyst component, said procatalyst component comprising magnesium,titanium and at least two internal donor compounds, at an elevatedtemperature in a reaction medium, in which a major part is formed bypropylene, whereby the polymerization is carried in at least one slurryreactor in the presence of liquid propylene at a polymerizationtemperature between 80-91° C. and by using a catalyst system in whichone of said internal donor compounds is slightly soluble and anotherinternal donor compound is easily soluble, the amount of said slightlysoluble donor compound in the catalyst system being at least 1 w-%. Inother words, if transesterification method is used, saidtransesterification reaction is carried out only partly.

Said internal donor compounds are preferably brought in the procatalystcomposition together with the titanium component. The titanation of theprocatalyst composition is carried out at least twice. During the firsttitanation the molar ratio of the added phthalic acid ester andmagnesium halide is preferably equal or greater than 0.1. During thesecond titanation the molar ratio of the added phthalic acid ester andmagnesium halide is 0-0.3. If during the second titanation no phthalicacid ester is added, then further titanation steps are not necessary.However, if phthalic acid ester is added during the second titanation,the third or possibly more titanation steps are necessary.

The procatalyst composition is used together with an organometalliccocatalyst, like aluminium trialkyl, and preferably with an externaldonor, such like cyclohexyl methylmethoxy silane or dicyclopentyldimethoxy silane.

The catalyst can also be prepolymerized prior to feeding intopolymerization reactor. In the prepolymerization the catalyst componentsare contacted for a short period with a monomer before feeding to thereactor.

The transesterification method provides a convenient way to bring in theprocatalyst composition at least one slightly soluble internal donor.However, any other methods can be used to bring in the procatalystcomposition at least one internal donor compound, in an amount of atleast 1 w-% is a slightly soluble internal donor compound.

The process described above makes it possible to produce polypropyleneshaving a molecular weight and melt index varying from low to very highand at the same time maintaining a high isotacticity index. A greateramount of polymer can be achieved by catalysts according to theinvention compared to the traditional catalysts or greater production byvolume can be achieved from the same reactor volume. The products havehigh elasticity or high crystallinity and high flexural modulus.

According to one embodiment of the invention the process comprises onlyone slurry reactor, which is operated at a temperature between 80° C.and the critical temperature of the reaction mixture. This means thatthe temperature is generally between 80° C. and 91° C. The pressure hasno upper limit, but for practical reasons the preferable pressures arein the range of 46-70 bar, preferably 50-70 bar.

The polymerization is carried out by feeding a catalyst system, amixture of propylene acting as reaction diluent and optional hydrogenand comonomer into the slurry reactor. The polymerization heat isremoved by cooling the reactor by cooling jacket. The residence time inthe slurry reactor must be at least 15 minutes, preferably 20-100 minfor obtaining a sufficient degree of polymerization. This is necessaryto achieve polymer yields of over 40 kg PP/g cat.

According to one embodiment of the invention, light inert hydrocarbonsare fed to the reactor. Examples of such hydrocarbons are iso-butane,n-butane and isopentane. The light, inert hydrocarbon in thepolymerization mixture lowers the pressure required in the reactor. Theincreased catalyst activity at relatively high temperature compensatesthe decreased activity due to lowered concentration of propylene.

If lower molecular weight polypropylene is the desired product hydrogencan be fed into the reactor. Hydrogen can be added in the reactor0.001-100 mol H₂/kmol propylene, preferably in the range of 1.5-15 molH₂/kmol propylene.

Comonomers can be added into the reactor in any desired amount,preferably 0-20% of the monomer feed. Ethylene, butylene and hexene,among others, can be used as comonomers for the manufacture of polymersfor blow molding sheets, pipe and film.

According to a preferable embodiment of the invention, it comprises twoslurry reactors, which are operated at a temperature of 80-91° C. Thedual reactor system is used because it decreases the possibility thatcatalyst particles move unreacted to the second reactor. This wouldcause gels or difficulties in downstream because of high catalystactivity. The pressure can be between within the range of 35-70 bar,while preferably it can be less, eg. 40-60, if light hydrocarbons areadded into the reaction mixture. Hydrogen can be present in the amountof 0-15 mol/kmol propylene feed, preferably 0-3 mol/kmol propylene.Because the polymerization temperature is high, the molecular weightdistribution tend to be narrow, but can be controlled broad in tworeactors by varying hydrogen concentration in different reactors. Theresidence time can be varied for example between 15-100 min such thatthe residence time in the second reactor can be the same or up to threetimes as that in the first reactor. This means that the reactor volumeof the second reactor can likewise be the same or up to three times asthat of the first reactor.

Hydrogen can be added in the second reactor at 0.001-100 mol H₂/kmolpropylene, preferably in the range of 1.5-15 mol H₂/kmol propylene. Theamount of hydrogen into the second reactor can be equal to or higherthan that of the first reactor.

According to one preferable embodiment of the invention, two sequentialloop reactors are used and the polymerization temperature in the firstreactor is lower than in the second reactor. The polyme rizationactivity of the catalyst decreases in the first loop reactor, but thiseffect can be compensated in the second reactor due to highertemperature.

Comonomers can be added into the first reactor and second reactor in anydesired amount, preferably 0-20% of the monomer feed. Ethylene, butyleneand hexene, among others, can be used as comonomers for the manufactureof polymers for blow molding, sheets, pipe and film.

By this way propylene polymers having a broad or bimodal molecularweight distribution can be produced. The polymers have a high flexuralmodulus of 1700-2100 MPa.

If polymers having a broad or very broad or bimodal molecular weightdistribution is desired, the slurry reactor or reactors can be followedby a gas phase reactor or reactors. By this way, higher comonomercontents can be used and multimodal products achieved. Thepolymerization in the gas phase can be carried out at a temperature of60-100° C. and in the pressure of 10-40 bar. It is desirable that nohydrogen or a minor amount of hydrogen is fed into the gas phasereactor. If hydrogen is applied, it is optionally removed from thereaction mixture before feeding the polymer into the gas phase reactor.This can be done by ordinary means, for example by cyclone separators orother suitable flash tank.

In this way high impact resistant products having a raised stiffness canbe produced.

The invention is further illustrated by the following non-limitingexamples.

EXAMPLES 1a-1d

The procatalyst used in Examples 1a-1d was made according to Finnishpatent 88048. Two titanations were carried out. In the first titanationthe ratio of donor (dioctyl phthalate) to magnesium was 0.15 (mol/mol).In the second titanation no donor was used. The procatalyst contained2.0 w-% of Ti, 15.2 w-% of Mg, 0.7 w-% of DOP (dioctylphthalate) and 6.2w-% of DEP (diethylphthalate).

Propylene was polymerized in stirred-tank reactor having a volume of 5l. About 0.6 ml triethyl aluminium (TEA) as a cocatalyst, ca. 0.2 ml of25 v % solution of cyclohexyl dimethoxy silane (CHMMS) in n-heptane asan external donor and 30 ml of n-heptane were mixed and allowed to reactfor five minutes. Half of the mixture was added to the reactor. Al/Timole ratio was 500 and Al/external donor was 20 (mol/mol). 70 mmol ofhydrogen and 1400 g of propylene were introduced into the reactor andthe temperature was raised within 15-30 minutes to a desiredpolymerization temperature. The polymerization time was 60 minutes,after which the polymer formed was taken out from the reactor.

The polymerization conditions used are presented in Table 1 below.

TABLE 1 Polymerization conditions Time to reach Poly- poly- merizationmerization temp- Amount of temp- erature catalyst TEA CHMMS eratureYield Example ° C. mg ml ml min g 1 a 70 24 0.68 0.2 15.5 773 1 b 80 200.57 0.17 23 1012 1 c 85 20.6 0.59 0.17 26 1018 1 d 90 20.7 0.59 0.17 331002

EXAMPLE 2a-2d

The procatalyst used in Examples 2a-d was made according to FI963707.Three titanations were carried out. In the first titanation the ratio ofthe donor (dioctyl phthalate) to magnesium was 0.15 (mol/mol). In thesecond titanation the donor/Mg ratio was 0.10 (mol/mol). In the thirdtitanation no donor was used. The procatalyst contained 2.5 w-% of Ti,14.8 w-% of Mg, 6.9 w-% of DOP and 6.2 w-% of DEP.

The polymerization was carried out as in Example 1.

TABLE 2 Polymerization conditions of Example 2 Time to reach Poly- poly-merization merization temp- Amount of temp- erature catalyst TEA CHMMSerature Yield Example ° C. mg ml ml min g 2 a 70 19.8 0.71 0.21 19.5 8252 b 80 16.9 0.60 0.18 23 944 2 c 85 18.5 0.66 0.19 25 916 2 d 90 21,00.75 0.22 28 941

EXAMPLES 3a-3d

The procatalyst used in Examples 3a-d was made according to FI963707.Three titanations were carried out. In the first titanation the ratio ofthe donor (dioctyl phthalate) to magnesium was 0.10 (mol/mol). In thesecond titanation the donor/Mg ratio was 0.175 (mol/mol). In the thirdtitanation no internal donor was added. The procatalyst contained 2.7w-% of Ti, 17.8 w-% of Mg, 11.9 w-% of DOP and 1.9 w-% of DEP.

The polymerization was carried out as in Example 1.

TABLE 3 Polymerization conditions of Example 3 Time to reach Poly- poly-merization merization temp- Amount of temp- erature catalyst TEA CHMMSerature Yield Example ° C. mg ml ml min g 3 a 70 19.7 0.76 0.22 17 10763 b 80 16.1 0.62 0.18 22 111O 3 c 85 17.8 0.69 0.20 25 1090 3 d 90 18.20.70 0.20 29 1027

Comparison Example 1a-1d

The comparison catalyst was prepared according to EP 0045 977.Essentially no transesterification took place at the titanationtemperature used in the preparation of this procatalyst. In other words,the added internal donor in the activated procatalyst was essentiallythe same also after the titanation treatment. The procatalyst contained2.4 w-% of Ti, 9.0 w-% of diisobutyl phthalate (DiBP) and only 0.1 w-%of transesterified donor DEP.

The polymerization conditions used are presented in the following Table4.

TABLE 4 Polymerization conditions used in comparative examples 1a-1dTime to reach Poly- poly- merization merization temp- Amount of temp-erature catalyst TEA CHMMS erature Yield Example ° C. mg ml ml min g 1 a70 15.9 0.54 0.16 15 604 1 b 80 18.0 0.62 0.18 24 840 1 c 85 18.I 0.620.18 25 757 1 d 90 17.9 0.61 0.18 28 495

The results of polymerizations are presented in Table 5. Melt index wasmeasured according to ISO 1133:1991. Isotacticity index was measured byextracting with boiling n-heptane for 1 hr in Kumagawa glass extractor.Isotacticity index is n-heptane insoluble material of polymer calculatedas w-%.

TABLE 5 Results of test polymerizations Poly- meri- zation temp-Activity Bulk Isotactic erature kgPP/ Activity MFR₂ density indexExample ° C. g cat h kg PP/g Ti g/10 min kg/m³ % 1 a 70 32.2 1610 5.6380 98.0 1 b 80 50.6 2530 8.9 410 97.5 1 c 85 49.4 2471 12.3 420 97.8 1d 90 48.4 2420 17.9 400 98.8 2 a 70 41.7 1667 4.5 430 97.3 2 b 80 55.92234 8.9 500 97.9 2 c 85 49.5 1981 11.9 490 97.5 2 d 90 44.8 1792 16.7510 97.4 3 a 70 54.6 2022 8.9 470 97.2 3 b 80 68.9 2553 14.8 470 97.4 3c 85 61.2 2268 22.1 470 97.0 3 d 90 56.4 2090 34.2 460 97.0 Comp. 1a 7038.0 1583 9.4 470 95.6 Comp. 1b 80 46.7 1944 15.1 470 96.2 Comp. 1c 8541.8 1743 22.8 460 95.6 Comp. 1d 90 27.2 1152 36.0 460 93.1

The results presented in Table 5 clearly show that with the catalystsaccording to the invention it is possible between temperature intervalof 80-90° C. to polymerize propylene with a catalyst activity remainingat rather high level, whereas with ordinary Ziegler-Natta catalysts theactivity decreases from about 46 kg PP/g cat h to 27 kg PP/g cat h. Theisotacticity index is with the inventional catalysts essentially at aconstant and high level in spite of increasing temperature, whereas withthe comparison catalyst the isotacticity index is at a lower level anddecreases when temperature increases. The melt index level withcomparison catalyst increases with temperature increasing, but thishappens at a cost of isotacticity index. Instead, with the inventionalcatalyst the melt index increases, but isotacticity index remains atabout the same level.

The following examples and comparison examples show that it is possibleto achieve more polymer from the same amount of catalyst according tothe invention.

EXAMPLE 4

A loop-loop cascade of volume 79 m³ is producing 150000 t/y meaning18.75 t/h. The loops have identical design with a volume of 39.5 m³,cooling area 244 m², diameter 0.6 m and specific area 5.67 m²/m³. Theoverall heat transfer coefficient is 1200 W/m²° C., which is typicalmeasured value in loop reactors with propylene bulk or propane slurry.The specific heat capacity of the cooling jacket water is 1.16 Wh/kg° C.and cooling water flow is 900 m³/h. Cooling capacity is not limiting thecases and therefore left out calculations.

The feed temperature is 25° C. containing 90% of propylene and the restpropane and possible hydrogen for MFR control. The minimum cooling waterinlet temperature to the cooling jacket is set to 37° C. This water iscooled by water with inlet temperature of 27° C. This gives a minimumtemperature difference of 10° C. The mileage (catalyst productivity) isbased on the formula Mileage=A*(residence time)^(B)(propyleneconcentration), and for coefficient A the value is 1531 and 2139 atactual polymerization temperatures 70° C. and 90° C. respectively, andthe coefficient B is 0.744. The heat of reaction is 557 Wh/kg.

EXAMPLE 4a

The conversion in loop-loop reactor cascade desribed above is calulatedto be 50% conversion in both reactors. The polymerization temperature is70° C. in the first reactor and 90° C. in the second. This meansproduction rate of 62.3 and 37.7% of total production in the first andsecond loop respectively. The total production is 18.75 t/h and therespective catalyst feed is 0.746 kg/h. The mileage is 39411 kgPP/kg catwith residence time of 91.2 min.

EXAMPLE 4b

The conversion in loop-loop reactor cascade desribed above is calulatedto be 35% and 50% conversion in the first and the second looprespectively. The polymerization temperature is 70° C. in the firstreactor and 90° C. in the second. This means production rate of 55.4 and44.4% of total production in the first and second loop respectively. Thetotal production is 18.75 t/h and the respective catalyst feed is 0.522kg/h. The mileage is 35876 kgPP/kg cat with residence time of 74.5 min.

EXAMPLE 5a

(Comparison)

The conversion in loop-loop reactor cascade desribed above is calulatedto be 50% conversion in both reactors. The polymerization temperature is70° C. in both reactors. This means production rate of 68 and 32% oftotal production in the first and second loop respectively. The totalproduction is 18.75 t/h and the respective catalyst feed is 0.555 kg/h.The mileage is 33816 kgPP/kg cat with residence time of 86.5 min.

Thus the mileage in Example 4a is 16.5% higher than in Comparisonexample 5a, meaning also respective direct catalyst savings.

EXAMPLE 5b

(Comparison)

The conversion in loop-loop reactor cascade desribed above is calulatedto be 35% and 50% conversion in the first and the second looprespectively and both at 70° C. Lowering the conversion in the firstloop is possible, eg. with increased propylene feed. This meansproduction rate of 68% and 32% of total production in the first andsecond loop respectively. The total production rate is 18.75 t/h and therespective catalyst feed is 0.623 kg/h. The mileage is 30126 kgPP/kg catwith residence time of 70.7 min.

Thus the mileage in Example 4b is 19.1% higher than in Example 5b. Alsoit is easier to reach a close 50/50 production split in Example 4b thanin the in comparison example 5b.

EXAMPLE 6

The conversion in the loop-loop cascade desribed above is calculated tobe 50% conversion in both reactors at 90° C. This means production rateof 68% and 32% of total production in the first and second looprespectively. The total production rate is 18.75 t/h and the respectivecatalyst feed is 0.463 kg/h. The mileage is 40483 kgPP/kg cat withresidence time 70.3 min.

What is claimed is:
 1. A process for homo or copolymerizing propylene,wherein propylene is polymerized in the presence of a catalyst systemcomprising a procatalyst component and a cocatalyst component, saidprocatalyst component comprising magnesium, titanium and at least oneinternal donor compound, at an elevated temperature in a reactionmedium, in which a major part is formed by propylene, wherein thepolymerization is carried in at least one slurry reactor in the presenceof liquid propylene at a polymerization temperature between 80° C. andthe critical temperature of the reaction medium and by using a catalystsystem where said internal donor compound is slightly soluble, theamount of said slightly soluble donor compound in the catalyst systembeing at least 1 wt %.
 2. A process according to claim 1, wherein saidprocatalyst component comprises magnesium chloride, a titanium compound,a lower alcohol and an ester of phthalic acid, wherein said loweralcohol and said phthalic acid ester has been reacted to form saidslightly soluble internal donor compound.
 3. A process according toclaim 2, wherein said reaction is a transesterification.
 4. A processaccording to claim 2, wherein the lower alcohol is methanol or ethanoland the ester is propylhexyl phthalate, dioctyl phthalate, di-nonylphthalate, di-isodecyl phthalate, di-undecyl phthalate, di-tridecylphthalate or di-tetradecyl phthalate, and the transesterification iscarried out at the temperature of 110-150° C.
 5. A process according toclaim 2, wherein said internal donor compound is diethyl phthalate.
 6. Aprocess according to claim 2, wherein said internal donor compound isbrought into the procatalyst component in titanation together with thetitanium compound.
 7. A process according to claim 6, wherein saidtitanation is carried out at least twice.
 8. A process according toclaim 7, wherein during the first titanation the molar ratio of addedphthalic acid ester and magnesium halide is equal or greater than 1 andduring the second titanation the molar ratio of added phthalic acidester and magnesium is between 0 and 0.3.
 9. A process according toclaim 1, wherein propylene is polymerized at a temperature between80-91° C. in the presence of a catalyst system comprising a procatalystcomponent and a cocatalyst component, said procatalyst componentcomprising magnesium, titanium and at least two internal donorcompounds, one of said internal donor compounds being slightly solubleand one of said internal donor compounds being easily soluble, wherebythe amount of said slightly soluble donor compound in the catalystsystem being at least 1 w-%.
 10. Process according to claim 1, whereinit comprises two loop reactors in series, where the polymerizationtemperature at the second reactor is higher than in the first reactor.11. Process according to claim 10, wherein light hydrocarbons are addedto one or both the reactors in order to decrease the polymerizationpressure.
 12. Process according to claim 1, wherein the catalyst has anactivity over 1500 kgPP/g Ti between said temperature interval.
 13. Aprocess according to claim 1, wherein the propylene homo- or copolymerhas an isotacticity of at least 97%, and the melt index being at least5.
 14. A process according to claim 1, wherein the catalyst componentsare precontacted before feeding into the reactor.
 15. A processaccording to claim 14, wherein the catalyst is prepolymerized withpropylene at a temperature of 0-80° C. before feeding into the reactor.16. A process according to claim 1, wherein comonomers are added intothe polymerization.
 17. Process according to claim 11, wherein the lighthydrocarbons are isobutane, n-butane or isopentane.
 18. A processaccording to claim 16, wherein the comonomers are ethylene or butylene.19. A process according to claim 1, wherein the internal donor compoundis slightly dissolved in a hydrocarbon solvent or compounds used as thecocatalyst.